The present invention relates to an appliance for use in video processing and more particularly relates to a technique of correcting the gray scale of a video luminance signal in an appliance for presenting a video.
A gray scale correction called xe2x80x9cgamma correctionxe2x80x9d has normally been carried out on a video luminance signal. This is a correction performed on a luminance signal using a gamma correction curve in accordance with the signal level thereof. The gamma correction is performed if the level of a luminance signal is equal to or greater than a predetermined value, which is referred to as a xe2x80x9ccorrection start pointxe2x80x9d. As is often the case with the prior art, the same correction start point is used for any luminance signal. Also, even if the correction start point is changed according to the average of a luminance signal, for example, the correction quantity is obtained only from the difference between the value of the luminance signal and the correction start point.
However, the characteristic value (e.g., maximum or average value) of a video luminance signal is greatly changeable from frame to frame. Accordingly, the correction quantity should be changed on a frame-by-frame basis even if a luminance signal with the same value is to be processed. For that reason, the gray scale correction could not be performed appropriately because the correction quantity has been obtained by using the same correction start point for any luminance signal or only the difference between the value of a luminance signal at an instant and the correction start point.
An object of this invention is providing a dynamic gamma correction apparatus for performing gray scale correction more effectively by obtaining an appropriate correction quantity for a specific type of video represented by a luminance signal with the characteristic (e.g., maximum or average value) of the video luminance signal taken into account.
Specifically, the present invention provides a dynamic gamma correction apparatus for performing a correction on a video luminance signal. The apparatus includes: a correction start point control section for obtaining a correction start point, which is a lower limit of a given correction range, in accordance with a characteristic value of the luminance signal; a gamma correction quantity calculating section for obtaining a provisional correction quantity for the luminance signal to be corrected using the correction start point and the luminance signal; a gamma correction gain control section for obtaining a gain for the provisional correction quantity in accordance with the characteristic value of the luminance signal; a multiplier for obtaining a product of the provisional correction quantity and the gain as a gamma correction quantity; and an adder for obtaining a sum of the luminance signal and the gamma correction quantity and outputting the sum.
According to the present invention, correction start point and gain with respect to provisional correction quantity are controllable in accordance with the characteristic value of a luminance signal. Thus, the gamma correction quantity, which is the final correction quantity, can be appropriate.
As used herein, the xe2x80x9ccharacteristic valuexe2x80x9d of a luminance signal refers to the maximum, average, minimum value or the like in a predetermined number of fields or frames of the luminance signal or to a value obtained from these values.
The inventive dynamic gamma correction apparatus may further include a maximum value detecting section for obtaining a maximum value of the luminance signal. And the correction start point control section preferably obtains the correction start point in accordance with the maximum value. In such an embodiment, a luminance range, in which the gamma correction is performed, is controllable in accordance with the maximum value of the luminance signal.
Alternatively, the inventive dynamic gamma correction apparatus may further include an average value detecting section for obtaining an average value of the luminance signal. And the correction start point control section preferably obtains the correction start point in accordance with the average value. In such an embodiment, a luminance range, in which the gamma correction is performed, is controllable in accordance with the average value of the luminance signal.
As another alternative, the inventive dynamic gamma correction apparatus may further include: a maximum value detecting section for obtaining a maximum value of the luminance signal; and an average value detecting section for obtaining an average value of the luminance signal. And the correction start point control section preferably obtains the correction start point in accordance with a difference obtained by subtracting the average value from the maximum value. In such an embodiment, a luminance range, in which the gamma correction is performed, is controllable in accordance with the maximum and average values of the luminance signal.
As still another alternative, the inventive dynamic gamma correction apparatus may further include a minimum value detecting section for obtaining a minimum value of the luminance signal. And the correction start point control section preferably obtains the correction start point in accordance with the minimum value. In such an embodiment, a luminance range, in which the gamma correction is performed, is controllable in accordance with the minimum value of the luminance signal.
As yet another alternative, the inventive dynamic gamma correction apparatus may further include a maximum value detecting section for obtaining a maximum value of the luminance signal. And the gamma correction gain control section preferably obtains the gain in accordance with a value obtained by subtracting the maximum value from a preset maximum luminance. In such an embodiment, the gain with respect to the provisional correction quantity obtained by the gamma correction quantity calculating section is controllable in accordance with the maximum value of the luminance signal.
As yet another alternative, the inventive dynamic gamma correction apparatus may further include an average value detecting section for obtaining an average value of the luminance signal. And the gamma correction gain control section preferably obtains the gain in accordance with a value obtained by subtracting a preset threshold value from the average value. In such an embodiment, the gain with respect to the provisional correction quantity obtained by the gamma correction quantity calculating section is controllable in accordance with the average value of the luminance signal.
As yet another alternative, the inventive dynamic gamma correction apparatus may further include: a maximum value detecting section for obtaining a maximum value of the luminance signal; and an average value detecting section for obtaining an average value of the luminance signal. And the gamma correction gain control section preferably obtains the gain in accordance with the average and maximum values so that the higher the average value, the smaller the gain and that the lower the average value, the larger the gain. In such an embodiment, the gain with respect to the provisional correction quantity obtained by the gamma correction quantity calculating section is controllable in accordance with the average and maximum values of the luminance signal. In particular, since the gain is increased if the average value of the luminance signal is low, the gamma correction can be much more effective.
In the inventive dynamic gamma correction apparatus, the gamma correction quantity calculating section may obtain the provisional correction quantity by a linear function using the luminance signal to be corrected as a variable. And the linear function preferably has a predetermined number of intervals where the greater a variable associated with one of the intervals, the greater a coefficient of the linear function associated with the interval. In such an embodiment, a linear function is used and the provisional correction quantity can be obtained easily. Also, the higher the luminance is, the greater the provisional correction quantity can be.
The inventive dynamic gamma correction apparatus may further include a maximum value detecting section for obtaining a maximum value of the luminance signal. And the gamma correction quantity calculating section preferably determines the respective intervals of the linear function by the correction start point and the maximum value. In such an embodiment, an appropriate provisional correction value can be obtained for the luminance signal with a luminance between the correction start point and its maximum value.